1. Field of the Invention
The present invention relates to an emitting device and a manufacturing method therefor. More particularly, the present invention relates to an emitting device including an emitting layer formed using a group IV semiconductor material and a manufacturing method therefor.
2. Description of the Related Art
At present, most semiconductor emitting devices are formed using a group III-V compound semiconductor material. This is because a group III-V compound semiconductor has a band structure of the direct transition type and high luminous efficiency may be obtained even at room temperature. On the other hand, with regard to a material having a band structure of the indirect transition type such as Si, the luminous efficiency is extremely low, and thus, reputedly, such a material is not suitable as a material of an emitting device. In recent years, attempts are being made to apply to an emitting device even such a material having a band structure of the indirect transition type such as Si, by confining electrons and holes in a region which is as narrow as several nanometers, thereby increasing the probability of emission recombination for generating light. In order to confine electrons and holes in a region of several nanometers in this way, a confinement structure having a high potential barrier (for example, equal to or larger than 300 meV) is necessary, and hence, heterojunction between materials having band gaps which are greatly different from each other is indispensable. In a group III-V compound semiconductor, while attaining lattice match, that is, based on stacking of good quality monocrystalline layers, heterojunction between materials having band gaps which are greatly different from each other may be formed. Examples include GaAs/AlGaAs. With the heterojunction, a quantum well or a quantum dot may be formed to form a confinement region (for example, a well or a dot) surrounded by a potential barrier, and electrons and holes may be confined therein.
On the one hand, with regard to a material based on Si which is a group IV semiconductor, there is no material which has a greatly different band gap and still may attain lattice match. For example, SiC (band gap: 2.2 to 2.9 eV, lattice constant: 4.3596 Å) the band gap of which is sufficiently larger than that of Si (band gap: 1.1 eV, lattice constant: 5.4309 Å) has lattice mismatch which is as high as about 20%, and hence, it is extremely difficult to form a good quality potential confinement region using heterojunction between monocrystalline layers. Meanwhile, with regard to the group IV semiconductor, in order to relax requirements of the above-mentioned lattice match, attempts are being made to form heterojunction with structurally highly flexible amorphous or polycrystalline structure for application to a solar cell or the like. In this case, the solar cell as a photoreceptor device may function, but, due to influence of recombination by many non-emission recombination centers which exist in amorphous or polycrystalline structure, efficiency sufficient for an emitting device cannot be attained.
Accordingly, as a method of forming a potential confinement region in a Si-based material or the like, a method is proposed in which fine crystals the size of which is several nanometers are formed in amorphous or polycrystalline region. This method is a method in which good quality fine crystals are formed in structurally flexible amorphous or polycrystalline structure, and the formed fine crystalline region has nothing existing therein that can be a non-emission recombination center such as a defect or an impurity. By confining electrons and holes in this narrow fine crystalline region, non-emission recombination is suppressed, and further, overlap of a wave function of electrons and a wave function of holes becomes large, and thus, emission recombination is promoted. These two effects enable enhancement of the luminous efficiency even if the material has a band structure of the indirect transition type. Further, in this case, electrons and holes are confined in the region of several nanometers, and hence, due to the quantum size effect, emission wavelengths thereof become shorter compared with those when there is no confinement. Therefore, if the size of the fine crystals may be controlled, design and control of the emission wavelength are possible.
As such a method, Japanese Patent Application Laid-Open No. 2000-77710 proposes an emitting device using Si fine crystals and a SiC polycrystalline structure and a forming method therefor. The emitting device in Japanese Patent Application Laid-Open No. 2000-77710 is formed so as to include an emitting layer in which Si or Ge fine crystals having a particle size on the order of nanometers are dispersed so as to be dot-like in the SiC polycrystalline structure. With regard to the forming method, the emitting device is formed by alternately depositing a SiC polycrystalline film and Si or Ge fine crystals by low pressure chemical vapor deposition or the like. Alternatively, a method is adopted in which, after a SiC polycrystalline film and a Si or Ge polycrystalline film are alternately formed, annealing is carried out in a vacuum or the like to form Si or Ge fine crystals.
The above-mentioned conventional example in Japanese Patent Application Laid-Open No. 2000-77710 has the following problem. The problem is described with reference to FIG. 1 for illustrating comparison between the present invention and the conventional example. In FIGS. 1A to 1D, FIG. 1A illustrates a structure of the conventional example, and FIGS. 1B and 1C conceptually illustrate a structure of the present invention. In the structure of the above-mentioned conventional example, as illustrated in FIG. 1A, Si fine crystals 102 exist in a SiC polycrystalline structure 104, and hence a border 106 between a fine crystalline structure and a polycrystalline structure is coincident with a border 106 between a well and a barrier in a potential confinement structure. This means that, while the crystallinity in a well region is satisfactory, the crystallinity on the border between the well and the barrier and in a barrier region is low. Here, Si is of the indirect transition type, and hence, in order to greatly improve the luminous efficiency, it is necessary to reduce the size of the potential confinement region to several nanometers, more specifically, on the order of 4 nm or smaller. When the well region is caused to be smaller in order to confine electrons and holes in such a small region, the wave functions of the electrons and the holes cannot exist only in the well region but extend greatly to the barrier region.
FIGS. 2A and 2B illustrate exemplary wave functions of electrons in the case of Si quantum well structures. FIG. 2A illustrates a wave function 202 in a case where the thickness of the Si quantum well is 1 nm and SiO2 having a large band gap (8 eV) is used for a barrier layer. Calculation by the present inventors has made it clear that, even in the case of such a large band gap, in an extremely thin region of, for example, 1 nm, the wave function extends to a part of the barrier region (about 1 nm). Similarly, FIG. 2B illustrates a wave function 204 in a case where the thickness of the Si quantum well is also 1 nm and SiC as a semiconductor material through which current may pass is used for the barrier region. In this case, the band gap is smaller than that of SiO2 and the potential barrier becomes lower, and thus, it is also clear that the amount of extension of the wave function to the barrier region becomes further larger (about 3 nm). In this case, the probability of existence of electrons and holes in the barrier layer is no longer negligible, and as a result, the ratio of recombination in the barrier layer becomes larger. Here, the low crystallinity in the barrier layer is reflected to cause non-emission recombination the recombination velocity of which is fast to occur on a priority basis, and, as a result, the luminous efficiency of the emitting layer as a whole is greatly lowered. Further, even if the size on average of the fine crystals may be controlled, it is extremely difficult to make uniform the size of all the fine crystals, and an emission spectrum from the fine crystals has a wide distribution. As a result, the half-width increases.